Phase-locked loop having charge pump controlled according to temperature and frequency

ABSTRACT

A phase locked loop (PLL) includes a phase comparator (12), a charge pump (13), a loop filter (14) and a voltage controlled oscillator (15). In order to minimize the lock-in time of the PLL under conditions of varying external temperatures, the PLL is supplied with temperature measurements, and the output of the charge pump is controlled according to both the external temperature and a desired frequency.

This application is a national phase of international applicationPCT/F195/00320, filed Jun. 5, 1995.

The present invention relates to a method for controlling a phase-lockedloop, which loop comprises a phase comparator, a charge pump, a loopfilter and a voltage-controlled oscillator, and in which method the loopis provided with information on the measurement result of externaltemperature.

The operation of phase-locked loops is sensitive to variations inexternal temperatures, as is usually the case with electronic componentsand devices. Phase-locked loops are used, for example, in base stationsand subscriber telecommunication terminals whose operation conditionsvary greatly.

Phase-locked loops are designed and their adjustment is carried out, ifpossible, in such a way that when a loop moves from one frequency toanother, the lock-in time of the loop onto the new frequency would be asshort as possible. Best results are obtained, if loop adjustment iscritically attenuated. In such a case, frequency error decreases fastestand the lock-in time is minimized. In phase-locked loops, variations intemperature cause the dynamic response of the loop to change. In casesthe dynamic response changes due to variations in temperature, theadjustment either becomes supercritical or subcritical which results inan increase in the lock-in time.

The aforementioned phenomenon is especially problematic in systems whichutilize frequency hopping, such as a cellular radio network, in whichthe frequency used changes time-slot-specifically. In the prior artsolutions, it is well known to implement frequency hopping as aso-called baseband hopping in which a frequency synthesizer remains on afixed frequency and does not change frequency time-slot-specifically. Incases frequency hopping synthesizers are used, the aim is to stabilizethe lock-in time by an analog coupling in which the amplification of theoperational amplifier of the loop filter is temperature controlled.

The drawback of baseband frequency hopping is that its implementation ina base station of a cellular radio network requires complete units andseveral transceivers in the base station. The number of hoppingfrequencies, however, is restricted to the number of transceivers. It isnot possible to apply said method in a subscriber telecommunicationterminal.

The dynamic response of a phase-locked loop is determined by a phasecomparator, a loop filter and a voltage-controlled oscillator all ofwhich are temperature dependent components. Furthermore, temperaturedependent behaviour varies from one frequency of the voltage-controlledoscillator to another. For this reason, it is very difficult to performtemperature dependent amplification adjustment of a loop filter by ananalog coupling, and to perform adjustment by both temperature andfrequency is virtually impossible.

It is thus an aim of the present invention to implement an adjustment ofa phase-locked loop, which adjustment stays critically attenuated in therequired temperature range. Thus, by a method in accordance with theinvention it is possible to minimize the lock-in time when moving fromone frequency to another regardless of variations in temperature.

This is obtained by a method described in the introductory section,characterized in that the output signal of the charge pump is controlledon the basis of external temperature and the frequency being monitored.

The invention further relates to a phase-locked loop which comprises aphase comparator, a charge pump, a loop filter and a voltage-controlledoscillator, and which loop has measurement information on externaltemperature as an input. It is characteristic of a phase-locked loop ofthe invention that the loop comprises a means for controlling the outputsignal of the charge pump on the basis of the external temperature andthe frequency being monitored.

The method of the invention enables temperature compensation of afrequency hopping synthesizer without external components. The method ofthe invention and the phase-locked loop can be used both in a basestation and in a subscriber telecommunication terminal. In the basestation equipment, the information on temperature required for theadjustment of the phase-locked loop is already readily available.

In the following, the invention will be described in greater detail withreference to the examples of the accompanying drawings in which

FIG. 1 shows a block diagram illustration of the structure of thephase-locked loop in accordance with the invention,

FIG. 2a shows a block diagram illustration of a charge pump used in aphase-locked loop of the invention,

FIG. 2b illustrates an output signal of a charge pump, and

FIG. 3 illustrates by means of a block diagram an implementation exampleof a control block of a charge pump used in a phase-locked loop of theinvention.

FIG. 1, then, illustrates the structure of a phase-locked loop. Thephase-locked loop is applied a reference frequency 10 as an input, whichreference frequency is fed through a reference divider 11 to a phasecomparator 12 to which a signal from a main divider 16 is applied as asecond input. The phase comparator compares the phase of the two inputsignals and, proportionally to the phase difference, controls a chargepump 13 by the two signals. The charge pump 13 generates a signalproportional to the phase difference and frequency, which signal isfiltered by a low pass filter 14 and applied as control to avoltage-controlled oscillator 15. The oscillator output signal is fedback to the phase comparator 12 through the main divider 16. The methodof the invention requires that a software controlled power source, suchas the circuits Philips UMA1005T, SA7025 or SA8025, and MotorolaMC145200 and MC145201, is used in the phase-locked loop.

FIG. 2a illustrates a charge pump of a phase-locked loop in greaterdetail. The charge pump comprises two power sources 23a and 23b to whichsignals 20 and 21 from the phase comparator 12, and operating voltages22a and 22b are applied as inputs. In a phase-locked loop of theinvention, the charge pump also comprises a means 24 which controls thepower sources 23a, 23b by signals 25a, 25b on the basis of a controlsignal 26. The control signal 26 can thus provide information on theexternal temperature. As an output of the charge pump, a pulse shapesignal 27 illustrated in FIG. 2b is provided, said pulse shape signal 27being proportional to the output signal of the phase comparator.

The output signal of the charge pump contains either negative orpositive pulses whose frequency and width are proportional to the outputsignal of the phase comparator. FIG. 2b shows two pulses 28, 29 of whichthe first pulse 28 is positive and the second pulse 29 negative. Lowpass filtered in the filter 14, the pulses control the output frequencyof the oscillator 15 to the right direction. In a method of theinvention, the amplitude of the output signal of the charge pump iscontrolled on the basis of information on external temperature. Thus,the phase-locked loop can adapt to variations in temperature.Temperature variations, then, influence the height of the output signalsof the charge pump, and the output signal of the phase comparatorinfluences the width and frequency of the pulses.

The response of an open loop of a phase-locked loop can, for example, beof the following kind: ##EQU1## in which α=control factor

K_(D) =output current/phase difference

T=temperature

s=jω, complex frequency

t₁ =t₂ =t₃ =time constants of the loop filter

K_(v) =frequency change/control voltage change of the oscillator

The control factor a in the form represents a correction factor for theinfluence of temperature. The variable s is frequency dependent, i.e.s=j2*π*f, in which f represents frequency.

In a preferred embodiment of a phase-locked loop of the invention, thecontrol factor is implemented by software. The charge pump can becontrolled by software so that the control information required by thecharge pump for each frequency and different temperatures isexperimentally sought beforehand and stored in a memory circuit, forexample, in a flash memory. The control information of the charge pumpcorresponding to different temperatures can either be stored in the samememory circuit as the data that determines frequency, or in a separatememory circuit reserved for temperature compensation.

FIG. 3 illustrates by means of a block diagram a preferred embodiment ofa control block of a charge pump used in a phase-locked loop of theinvention. The control can be implemented in several other ways, too.However, in the described method a simple structure is obtained.Typically, it is enough in temperature control that in a desiredtemperature range (for example, -10° C.-+60° C.), the temperature isincluded in a certain temperature segment which can be set to suit thepurpose, for example <0° C., 0° C.-20° C., 20° C.-40° C. and >40° C.Control to the charge pump is transmitted depending on the segment towhich the measured value of temperature at a given time belongs.

Information 30 on external temperature is applied to the control blockas an input, for example, as a temperature dependent voltage whichtypically lies in the 0 . . . 5 V area. The control block comprises ameans 31 which interprets the temperature message and detects whichtemperature range is in question. The control block further comprises acoding means 32 which codes the information on the temperature range asan address of the memory circuit 35. An advantageous embodiment of thecontrol block further comprises means 33 and 34 which prevent thetemperature control from altering in cases the memory circuit outputcontrols the charge pump. By way of synchronization, address informationchanges can be performed at a suitable time, i.e. when the memorycircuit is not being read.

Address information 36 from the coding means is thus applied as an inputto memory circuit 35 as the most significant bits. As the second inputto the memory circuit, the address information 37 is applied, whichchooses the output data as a function of frequency. In the example ofthe figure, the charge pump control information corresponding todifferent temperatures is thus stored in the same memory circuit as thefrequency determining data. The output 35 of the memory circuit, then,provides a control signal 38 for the charge pump, which signal is thusdependent on both the frequency information and the externaltemperature.

The memory circuit 35 is divided in the example of FIG. 3 into fourtemperature blocks on the basis of two most significant address buses.The blocks are identical as far as their division number data isconcerned, but each channel (frequency) can have a separate charge pumpdata.

Although the invention is described above with reference to the exampleof the attached drawings, it is obvious that the invention is notrestricted to that, but it may be varied in various ways within theinventive idea of the attached claims. Temperature compensation, forexample, can be performed as a combination of analog and softwareadjustment in which case the analog compensation could function as afine adjustment for the software performed rough adjustment.

We claim:
 1. A method for controlling a phase-locked loop, which loopcomprises a phase comparator (12), a charge pump (13), a loop filter(14) and a voltage-controlled oscillator (15), and in which method theloop is provided with information on the measurement result of externaltemperature, characterized in that the output signal of the charge pump(13) is controlled according to the external temperature and a desiredfrequency.
 2. A method as claimed in claim 1, characterized in that theamplitude of the current output of the charge pump (13) is controlled onthe basis of information on external temperature.
 3. A method as claimedin claim 1, characterized in that in the phase-locked loop, controlinformation of the charge pump (13) corresponding to differenttemperatures is stored in a memory circuit (35), and that themeasurement result of the temperature applied as an input to the loop iscoded as the memory circuit address on the basis of which the chargepump is provided with the desired control.
 4. A method as claimed inclaim 1, characterized in that the desired temperature range is dividedinto several temperature segments, and the temperature control of thecharge pump (13) depends on the segment to which the externaltemperature belongs.
 5. A phase-locked loop which comprises a phasecomparator (12), a charge pump (13), a loop filter (14) and avoltage-controlled oscillator (15), and which loop has measurementinformation on external temperature as an input, characterized in thatthe loop comprises a means (24) for controlling the output signal of thecharge pump on the basis of the external temperature and a desiredfrequency.
 6. A phase-locked loop as claimed in claim 5, characterizedin that the means (24) for controlling the output signal of the chargepump comprises a means (31) for processing the temperature measurementresult, a means (32) for coding the temperature information as a memorycircuit address, and a memory circuit (35) in which the charge pumpcontrol information corresponding to different temperatures is stored.7. A phase-locked loop as claimed in claim 6, characterized in that themeans (24) for controlling the output signal of the charge pumpcomprises means (33, 34) which prevent the address of memory circuit(35) from altering in cases the memory circuit output controls thecharge pump (13).